The present invention relates to a compressed air throttle apparatus in particular used for powder spraycoating equipment. Moreover the present invention relates to powder spraycoating equipment containing at least one such throttle apparatus.
Powder spraycoating equipment comprising a throttle apparatus of the above kind is known from the European patent document EP 1 156 882 B1. It comprises an electrical stepping motor which rotates a valve element by the intermediary of a bellows connection. The valve element is fitted with a thread engaging a housing thread whereby, during its rotation, said valve element is axially displaced relative to a valve seat in order to change the aperture of a throttling duct present in the valve seat. Said patent also shows a throttle apparatus having two throttle valve s configured in mutually opposite manner and being driven by the same stepping motor, as a result of which, during opening one throttle valve, the other throttle valve shall close or, vice-versa depending on the direction of rotation of said stepping motor. The stepping motor shall be rotated by a given number of steps from its reference position to a predetermined aperture of the minimum of one throttling duct.
In practice, the known throttle apparatus valve is at its minimum aperture in the reference position, said minimum aperture being at least completely closed or at most a slightly open one to a compressed air leakage flow that is measured before operating the throttle apparatus and that is taken into account when the stepping motor is electrically controlled to adjust a desired operational, compressed air flow. On account of manufacturing tolerances and the need to take into account the motor shaft angular position at the end of a rotational step, It is exceedingly difficult in practice to make use of the completely closed position of the throttle apparatus as the reference position from which the number of steps of said stepping motor shall be counted in order to allow a given airflow through the throttle apparatus valve.
FIG. 1 of the appended drawings shows a state of the art embodiment mode of spraycoating system defined in the said EP 1 156 882 B1 patent document. An electric stepping motor 2 is driven by an omitted electrical control in order to rotate by means of a bellows connector 4 a valve element 6 by a predetermined number of rotational steps for the purpose of adjusting thereby a valve needle tip 8 of the valve element 6 relative to a valve seat 10 and thus to adjust the aperture of a throttling duct 12 constituted in this valve seat 10. The valve element 6 is fitted with a thread 14 engaging a thread 16 of a housing 17, thus transforming the rotational displacement of the stepping motor 2 into an axial displacement of the valve element 6. At the minimal and preferably zero aperture of the throttling duct 12—such full closure of the throttling duct however being very difficult to attain in practice—further rotation and hence further axial displacement of the valve element 6 is stopped by stop 18 of the valve element 6 coming circumferentially to rest against a stop 20 of the housing 17. To allow opening the throttling duct 12 by more than a rotation of 360° of the valve element 6, the two stops 18 and 20 already must already be spatially apart far enough as indicated in FIG. 1 that they may be rotated past one another. This requirement entails an axially very short overlap of the two stops 18 and 20 at the minimal setting as a reference position of the aperture of the cross-sectional aperture of the throttling duct 12 and moreover a thread 14, 16 of relatively high pitch. The larger the thread pitch, the larger however the axial displacement of the valve element 6 per step of the stepping motor 2. Accordingly fine adjustment of the throttle apparatus valve 8, 10, 12 is precluded. This difficulty is compounded by the manufacturing tolerances of the particular constituents. On the other hand highly accurate adjustment of flows of compressed air through the throttling duct 12, further the ability to set minute changes in such a compressed air flow, are desirable. But the system of the state of the art already may incur an error in adjustment in that, when the two stops 18 and 20 make rotational contact, the stepping motor 2 has not yet fully carried out the rotational step required by its electrical control.